The present invention relates to a low frequency audio conversion circuit for enhancing the presence of low frequency audio by compensating the low frequency component which is deficiently reproduced by a speaker having a poor response for low frequency audio.
The size and shape of a speaker built in an ordinary television receiving set, etc. is restricted by the structure of receiver set, etc. Due to such restrictions, the reproduction of the low frequency sound is limited. Although satellite broadcasting programs and the Hi-vision programs are presented in digital sound, listeners are not able to enjoy the quality music source. In order to improve sound quality, some receiving sets available in the market are equipped with low-frequency dedicated speakers and amplifiers. However, the size of such sets inevitably increases because additional space for incorporating the dedicated speakers and amplifiers is needed. Moreover, such sets often have the problem of sound resonating within the cabinet of the television set.
To overcome these disadvantages, there have been circuits proposed for compensating the low frequency component of an audio signal one of which is shown in FIG. 6.
An exemplary circuit for low frequency compensation is described below with reference to FIG. 6.
In FIG. 6, numeral 10 denotes a low pass filter for extracting the low frequency audio component by passing only the low frequency signals. A normalizer 11 coordinates the amplitude of the low frequency component with that of other frequency components. A second harmonics generator 12 generates secondary harmonics. A third harmonics generator 13 generates tertiary harmonics. A n-th harmonics generator 14 generates n-th harmonics. An adder 15 adds signals from the harmonics generators.
The operation of the low frequency audio component compensation circuit is now described. When an audio signal comes to the circuit, only the low frequency component is extracted by low pass filter 10 and sent to normalizer 11. The normalizer 11 applies a normalization process to the low frequency signal component by coordinating the amplitude of the low frequency component with that of signals in other frequency ranges. The output of normalizer 11 is provided to second harmonics generator 12, third harmonics generator 13, and so on up to the n-th harmonics generator 14 which generates higher order harmonics. In each of the harmonics generators 12-14, the harmonics are generated through the following mathematical processing:
secondary harmonics are obtained according to EQU cos2 .THETA.=2cos.sup.2 .THETA.-1
where the original sound: is -cos .THETA.,
tertiary harmonics are obtained according to EQU cos3.THETA.=4cos.sup.3 -3cos.THETA.,
and fourth harmonics are obtained according to EQU cos4.THETA.=8cos.sup.4 .THETA.-8cos.sup.2 .THETA.+1.
Likewise, n-th order harmonics can be determined with similar mathematical formulas. The harmonics of each order are multiplied by a coefficient and then mixed at adder 15 with the original audio input signal to produce a final output signal
By using the low frequency audio component compensation circuit described above, even a speaker having poor low frequency reproduction characteristics as shown in FIG. 3(a) can reproduce the harmonics. As shown in FIG. 3(a), an audio component at frequency f0 is hardly reproducible because the frequency curve 7 of the speaker does not cover, for example, the low range component 8 at frequency f0. The speaker can reproduce, as compound sound, the secondary harmonics 9, tertiary harmonics 16 and the n-th order harmonics 17 of the low frequency audio component at frequency f0, as shown in FIG. 7. The compound sound containing secondary harmonics 9, tertiary harmonics 16 and the n-th harmonics of the low frequency audio component at frequency f0 creates, by taking advantage of the psychological effects of sound on the human auditory sense, an effect as if the low range sound is actually reproduced.
The conventional circuitry described above is difficult to implement with an analog circuit because of the need for mathematical processing. Another problem is that the scale of the circuitry becomes very large. As a matter of course, the conventional circuit is easily implemented by using digital signal processors. However, in this case a large processing program is required, which substantially increases cost.
Moreover, the conventional circuit described above has a drawback in that it eventually produces a malaise in the musical sense. For example, when a 55 Hz sound "la" is inputted, its secondary harmonic is a 110 Hz sound "la", but the tertiary harmonic is a 165 Hz sound which is almost "mi". The synthetic chord of these harmonics and the original sound creates a dissonance. Therefore, the conventional circuitry can only reproduce a sound in which the fidelity to the pitch of original sound is low.